This invention relates to audio high fidelity cables or transmission lines wherein the wavelength of signals carried on the transmission lines are generally longer than the length of the transmission lines, such as in audio signal and high fidelity sound reproduction applications. The invention has particular application where the range of frequencies is greater than several octaves and therefore wherein spurious oscillations (ringing) and broadband random noise carried on the transmission line can have potentially significant impact on the fidelity of a complex signal carried by the transmission line.
In contrast to long transmission lines where the impedance of the cable is matched to the impedance of the termination loads, it is conventional in short transmission lines to reduce the impedance, and more particularly the resistance, of the cable to a minimum to reduce the electrical resistive loss in the cable. When used in high fidelity audio signal interconnection cabling, as between components in an audio component system, such an approach introduces secondary problems, such as audible enhancement of the "brightness" frequencies (1000 Hz to 2000 Hz range) and can cause distortion in the desired audio frequency signals. The primary causes of these effects appear to be radio-frequency noise and spurious oscillation (ringing) in the interconnect cables.
It is common practice in low-signal-level audio frequency circuitry to provide a shielding sheath surrounding signal conductors between subsystems and within components of an audio system. A typical configuration is a twisted pair of conductors surrounded by a nonferromagnetic foil or braided sheath which is terminated at one end of the cable. This type of shielding exploits the Faraday Effect and is adequate for minimizing interference from sources external to the shield.
Noise which is internally generated in audio equipment can couple into the terminated shield surrounding the signal cable, which can cause a spurious oscillation (ringing) of an electromagnetic nature in the shield itself. The added noise and resultant ringing on the shield can couple to the signal leads, primarily by capacitive coupling between the shield and the signal leads. This coupling is aggravated where the length of the shield and the length of the signal lead are substantially equal. Thus the natural length-wise mode of ringing in each element is nearly identical, which can result in efficient tuned coupling between the shield and the leads. This is an undesirable condition.
It appears further that radio frequency sources can induce distortion in audio signals carried over leads of a shielded cable. The sources may be either internal (in an amplifier for example) or external (in a television set). The effects of radio frequency noise on audio frequency signals appear to be most pronounced at the higher audio frequencies, where distortion has been noted. The presence of oscillations and modal resonances in cables is believed to result in undesired enhancement in the "brightness" frequencies (1000 Hz 2000 Hz range). These effects are undesirable if accurate reproduction of recorded music and the like is desired.